Generally tire wear leads to lowered draining performance and longer braking distance on wet road surfaces. Also, in the case of winter tires, wear can bring about a marked drop in grip performance on an icy or snow-covered road. Furthermore, excessive wear can be very dangerous by allowing water to seep into the tread belts, which can eventually cause a rupture of the tire. Small vehicles are so arranged that when the remaining groove depth of a tire is reduced to 1.6 mm, a rubber projection called a “slip sign” makes its appearance in the tire groove. To ensure running safety of a vehicle, tires should be replaced before the appearance of the slip sign, but there are, in fact, many drivers who are least interested in such maintenance practice.
Thus, there is demand for technologies for automatically detecting wear of tires to give a warning to the driver. Also, from the viewpoint of vehicular control, it is desired that control for greater safety be realized through the grasp of changes in tire characteristics resulting from wear.
A known method for estimating wear of a tire is such that the absolute speed of a vehicle is calculated using GPS, an optical sensor or the like, then the dynamic radius of the tire is calculated by comparing it with the rotational speed of the wheel, and the amount of tire wear is derived from the difference between the dynamic radius of the tire and the radius of the tire when it is new (refer to Patent Documents 1 and 2, for example).
However, even for a completely worn tire, the difference in rotational speed between the worn tire and the new tire is about 1% at most. It has therefore been difficult to realize a constantly accurate estimation of tire wear from the difference in tire radius because of the necessity not only to measure with great accuracy but also to take into consideration various error factors in vehicular running such as the inner-outer wheel errors in cornering, errors due to acceleration slips at braking or drive, and errors due to inclinations of the road surface.
Also, other methods proposed for estimating tire wear may be such that a transponder or an IC tag is embedded in the tire tread together with a receiver installed on the vehicle body and tire wear is estimated using the loss of response as the transponder or the IC tag is broken or falls off when the tire is worn (refer to Patent Documents 3 to 5, for example) or that a detector consisting of a magnetic material or a conductive rubber is embedded in the tire tread together with a sensor installed on the vehicle body and tire wear is estimated by detecting the change in the signals detected by the sensor as the detector wears off as a result of tire wear (refer to Patent Documents 6 and 7, for example).    Patent Document 1: Japanese Unexamined Patent Application Publication No. 6-278419    Patent Document 2: Japanese Unexamined Patent Application Publication No. 7-164830    Patent Document 3: Japanese Unexamined Patent Application Publication No. 10-307981    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2004-205437    Patent Document 5: US2002/0116992A1    Patent Document 6: Japanese Unexamined Patent Application Publication No. 2003-214808    Patent Document 7: Japanese Unexamined Patent Application Publication No. 2005-28950